The field of this invention is reflector antennas or lens antennas for producing multiple beams, for example simultaneous multiple beams, without mechanically relocating the feed elements. Conventional feed systems for multiple beam antennas have often used horn antennas as feed elements. The horn antennas are clustered near the focal point of the main reflector or lens, whichever is used, and each horn antenna produces a primary radiation pattern illuminating the main reflector from a different point. This results in the multiple secondary beams which radiate into the remote field from the antenna as a whole.
An important objective in designing multiple beam antennas often is to obtain a beam spacing in the remote field of approximately one-half the beamwidth of an individual beam, where the term beamwidth is used here to indicate the spacing between halfpower points of an individual beam. When this design objective is obtained, the crossover loss at a beam crossover point, which is where the radiation pattern of one beam intersects the radiation pattern of an adjacent beam, is preferably about three decibels.
The beamwidth of each of the multiple beams in the remote field is determined primarily by the size of the main reflector expressed in wavelengths of the radiation. Consequently, for a fixed size of main reflector, the widths of individual beams in the remote field are fixed, and the beam crossover loss is determined almost entirely by the angular spacing between adjacent beams in the far field. This angular spacing between adjacent beams is in turn determined principally by the mechanical spacing between phase centers of the individual feed elements.
A troublesome problem of the prior art in producing a feed system for a multiple beam antenna is the difficulty of bringing the phase centers of the bulky individual feed elements close enough together while still maintaining efficient primary illumination patterns for the main reflector or lens.
A very desirable primary radiation pattern for illuminating a reflector or lens is a radiation pattern shaped like a piece of pie, that is, one which is a sector of a circular disk. This idealized shape of pattern has uniform radiation intensity throughout an angular sector, and zero intensity outside of that angular sector, which implies straight steep sides defining the edges of the pattern. In three dimensions the pattern has conical sides.
When horn antennas are used as the primary feeds in a multiple beam antenna, it is found that if the horns are made small enough to be mounted close enough together to obtain a close angular secondary beam spacing in the remote field, the primary radiation patterns from the individual horn elements are too broad to provide efficient illumination of the main reflector. This results in degraded over-all antenna efficiency. Either a significant amount of energy from the horns passes around the edge of the main reflector, or else, when the reflector is made large enough to intercept most of the primary energy from the feed horn, the main reflector is excessively large and expensive and/or the beamwidth in the remote field is so narrow that adjacent beams cross over at a point whose power level on either beam is weaker than the desired 3 dB crossover level. Alternatively, when the individual horn elements are each made large enough to provide good illumination of the main reflector, the horns must be spaced so far apart because of their size that their phase center spacing is too large to obtain close secondary beam angular spacing, with its attendant low crossover loss.